A previously developed statistical mechanical model for the chain melting phase transition in lipid bilayer membranes (R.E. Jacobs et al., Proc. Nat. Acad. Sci. USA 10, 3993 (1975)) is being extended to include phospholipids with ethanolamine as well as choline head groups and with trans- and cis-unsaturated double bonds in the hydrocarbon chains as well as saturated chains. A model for phase diagrams and phase equilibria in two component phospholipid bilayers (R.E. Jacobs, B.S. Hudson, and H.C. Andersen (to be published) will be applied to study mixtures of such phospholipids. Statistical mechanical models for the interaction of a lipid bilayer with an ionic solution are being developed to describe pH-induced and Ca ions-induced phase transitions and phase separations. BIBLIOGRAPHIC REFERENCE: "A Theory of the Chain Melting Phase Transition of Aqueous Phospholipid Dispersions," R.E. Jacobs, B. Hudson and H.C. Andersen, Proc. Nat. Acad. Sci. USA 72, 3993 (1975).